BetterTouchTool should now work on OS X Yosemite beta. Please redownload if you already have the beta installed.
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BetterTouchTool

BetterTouchTool is a great, feature packed FREE app that allows you to configure many gestures for your Magic Mouse, Macbook Trackpad and Magic Trackpad. It also allows you to configure actions for keyboard shortcuts, normal mice and the Apple Remote. In addition to this it has an iOS companion App (BTT Remote) which can also be configured to control your Mac the way you want.

BetterTouchTool includes many goodies, like window snapping or an integrated window switcher.

BTT Remote
BTT Remote is the iOS companion app for BetterTouchTool (you only need this if you want to remote control BetterTouchTool with your iPhone, iPad or iPod Touch) It's available for only $1.99 in the App Store!

Rob Griffiths, Macworld.com

For my needs I've found that this combination of the Magic Trackpad, BetterTouchTool and some utilities is the perfect mouse replacement

Tuaw.com

BetterTouchTool makes multi-touch infinitely more useful, for free.

Lifehacker.com

BetterTouchTool adds tons of new gestures to your mouse, such as tapping in the corner, tapping with multiple fingers, swiping in different directions, and, in the case of the Magic Mouse even pinching and stretching to zoom in and out.